A pipeline must, to some degree, follow the contour of the land through which the pipeline is laid. This is particularly true with underground pipe, which is becoming more prevalent. For example, a pipe passing under a ravine must often have appropriate bends to accommodate the ravine. In addition, with the increasing density of pipelines crossing the country, it is sometimes necessary for a section of a pipeline to be bent to avoid interfering with another pipeline.
Pipe bending machines have been developed that permit bending pipe to a desired degree. Examples of such pipe bending machines are machines disclosed in U.S. Pat. No. 5,092,150 issued on Jul. 19, 1991 to Cunningham and U.S. Pat. No. 5,123,272 issued on Sep. 30, 1991 to Heaman. As shown in these patents, the pipe is held in the bending machine by a pin up shoe and stiffback. The pin up shoe is positioned up and down by a wedge actuated by a hydraulic cylinder. The purpose of the wedge is to provide a mechanical advantage to the cylinder because the pin up shoe must restrain one end of the pipe during bending. The stiffback is positioned on the opposite side of a die and is raised and lowered by hydraulic cylinders. The stiffback bends the pipe around the radius of the fixed die, which acts as a fulcrum.
In general, as a pipe is bent, the outer part of the bend is stretched and the inner section of the bend is compressed. As a result of these opposite and unequal stresses, the pipe tends to distort, flatten, buckle, or even collapse, thereby destroying the utility of the pipe. Buckling occurs when the resistance to bending of the pipe becomes greater than the resistance to buckling. In addition, distortion is especially prevalent in large diameter, high strength, thin wall pipe that is bent in a cold condition and is commonly used in the pipeline industry.
Over the years, the tensile strength of steel pipe has been increased to allow the use of thinner wall pipe in the construction of pipelines. The reason for this change is the savings realized from the reduced amount of total steel required. However, it is well known that thin wall pipe distorts and buckles more easily than thicker wall pipe. Distortion and buckling are unacceptable. The most common location for distortion and buckling to originate is in the portion of the pipe adjacent to the last one third of the die.
To prevent distortion and buckling, the wall of the pipe must be supported in some manner during the bending operation to support the bend and thereby minimize the adverse effect of the opposite and unequal stresses induced during bending. Conventionally, this support has been in the form of a filling material or an internal mandrel that supports the inner wall. However, using a filling material or an internal mandrel is often not advantageous or practicable for many pipeline applications.
For example, using a filling material often requires filling the interior of the pipe with a combination of low melting point metals, such as bismuth, lead, tin, and cadmium, so that the pipe can be bent as a solid rod and the filling material can be melted away after bending. It may be impracticable to perform this operation for large diameter pipes because of the large quantities of filler materials and the long processing time that would be required. Furthermore, use of such filler materials may adversely affect the material properties of the pipe, such as corrosion resistance or strength.
In another example, using an internal mandrel requires selecting a mandrel specifically adapted for a particular range pipe bend radii, pipe diameters, pipe wall thicknesses, and pipe materials. The mandrel helps hold the pipe cross section round during the bend. The mandrel also typically has buckle-resisting strips that support the wall of the pipe during bending. Different mandrels must be selected as different types of pipe are bent to different radii, thereby increasing cost and delaying the pipe bending process. In addition, the mere process of inserting and aligning an internal mandrel is time consuming and also increases costs and delays the pipe bending process. Even when using an internal mandrel during bending, it is often difficult to hold the cross section of a pipe round during bending, which can also be detrimental to the corrosion coatings normally applied to the pipe. Also, distortion, flattening, buckling, and collapsing of the pipe may nevertheless occur even if an internal mandrel is used because an internal mandrel may not adequately support the bend.
Therefore, there is a need to develop technology for an improved means for supporting the bend of a pipe during pipe bending to minimize or prevent distortion, buckling, flattening, or collapsing of the pipe.